1. Field of the Invention
Illustrative embodiments of the present disclosure relate to a thin-film actuator, a liquid ejection device that ejects droplets of liquid, e.g., ink using the thin-film actuator, and an image forming apparatus that forms an image using the liquid ejection device.
2. Description of the Background
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having two or more of the foregoing capabilities. As one type of image forming apparatus using a liquid ejection device, inkjet recording apparatuses are widely used because of advantages such as nearly noiseless operation, high-speed printing, greater flexibility of ink selection, and availability of recording sheets (e.g., plain sheets) at reasonable prices.
A liquid ejection device used in such an inkjet recording apparatus has nozzles through which ink droplets are ejected, liquid chambers (also referred to as ejection chambers, pressurized chambers, pressure chambers, or ink channels) that communicate with nozzles, and pressure generators for ejecting ink stored in the liquid chambers.
One type of pressure generator is a bubble-type (thermal-type) pressure generator that ejects ink droplets by generating bubbles by film-boiling of ink using an electro-thermo transducer, e.g., a heat-resistant body disposed in the liquid chamber. Another type is a piezo-type pressure generator that ejects ink droplets by deforming/displacing a diaphragm forming a wall of the liquid chamber using an electro-mechanical transducer such as a piezoelectric element. The piezo-type pressure generator comes in several types, including, e.g., a vertical-vibration type deforming in the d33 direction, a lateral-vibration type (bend mode type) deforming in the d31 direction, and a shear-mode type using shear deformation.
With recent advances in the fields of semiconductor processing and micro-electro-mechanical systems (MEMS), thin-film actuators have been proposed in which liquid chambers and piezoelectric elements are directly built into a silicon (Si) substrate. For example, one approach like that described in Japanese Patent Application No. 2008-047689 proposes a piezo-electric actuator including a diaphragm and a piezoelectric element for deforming the diaphragm. To provide an inkjet recording head of high density and high precision at a reduced cost, the approach proposes to reduce a residual stress in the diaphragm by doping the diaphragm with impurities. A lead zirconate titanate (PZT) film, serving as a piezoelectric element, is a thin film of a thickness of 5 μm formed by sputtering, and the diaphragm is made of oxidized Si film. In this approach, introducing germanium (Ge), lead (P), boron (B), and/or other substances as impurities into the oxidized Si film is proposed, and an appropriate doping amount is assumed to be 35 mol % or lower.
Although the above-described approach may be effective in reducing stress on the diaphragm, it fails to provide a way to increase the generation force to achieve higher densities.
Further, consistent displacement of the piezoelectric element is crucial to good printing performance. To reduce fluctuation or variation in the displacement of the piezoelectric element in the thin-film type of piezoelectric actuator, another approach proposes to form an inter-layer insulation layer with an opening that defines the deformable area of the piezoelectric element and to manufacture a piezoelectric element deformable only within the opening. In this approach, a PZT film serving as the piezoelectric element is a thin film of a thickness of 4 μm formed by sputtering, and the diaphragm is an Si film of a thickness of 4 μm.
Such a configuration may reduce unwanted electrostatic volume and limit the deformable area of the piezoelectric element, thereby reducing a fluctuation or variation in displacement of the piezoelectric element to some extent. However, this approach also fails to provide a way to increase the generation force to achieve higher densities.
In still another approach, in a thin-film piezoelectric actuator the piezoelectric element is set to a thickness of 5 μm or less and the width in the short direction of the diaphragm is set to 160 μm or less. Although such a configuration may create the generation force needed to achieve current levels of density, this approach also fails to provide a way to increase the generation force to achieve higher densities.
More specifically, the conventional thin-film piezoelectric actuator has a single-layer piezoelectric structure including only one piezoelectric layer, i.e., a structure formed with a lower electrode, a piezoelectric layer, and an upper electrode. In such a configuration, to increase the generation force of the actuator, both the thickness of the piezoelectric element and the width in the short direction of the diaphragm should be optimized according to the thickness of the diaphragm. Accordingly, increasing the actuator density may cause a significant reduction in the width in the short direction of the diaphragm. For example, in the case of 600 dpi, the width in the short direction of the diaphragm is approximately 30 μm, and in the case of 1,200 dpi, the width is approximately 15 μm. In such cases, to obtain a desired generation force, the thickness of the diaphragm should be extremely thick (e.g., approximately 7 μm or more) at a markedly thin region of the piezoelectric layer. If a piezoelectric actuator having a single piezoelectric layer as described above is manufactured in such a region, the small thickness of the diaphragm results in a low yield. Further, the great thickness of the piezoelectric layer causes an increase in driving voltage (e.g., reaching approximately 70V at the thickness of 7 μm of the diaphragm, depending on film properties), which is far from practical use.